Regulated small current source



Aug. 27, 1968 J. R. YEAGER REGULATBD SMALL CURRENT SOURCE Q m 6 SI m m g M I 4 m T m \SQQSw R A mm om w 0 mmcfibomm 0 5 mm n N mm \m v A IF Q mmzv wm M mm United States Patent 3,399,339 REGULATED SMALL CURRENT SOURCE John R. Yeager, Cleveland Heights, Ohio, assignor to Keithley Instruments, Inc., Cleveland, Ohio, :1 corporation of Ohio Filed Nov. 2, 1964, Ser. No. 407,962 Claims. ((31. 323-22) ABSTRACT OF DISCLOSURE The present small current source includes a regulated power supply having a nominal output voltage, and a plurality of high resistance non-precision output resistors, each having a nominal resistance, arranged to be selectively connected individually in series with the power supply output. The power supply has a differential amplifier and a voltage divider connected across the power supply output and having an intermediate tap Whose potential controls the amplifier gain, and thus the output voltage of the power supply. A plurality of compensating resistors, one for each output resistor, are arranged to be selectively connected individually in the volt-age divider at one side of the intermediate tap. Each compensating resistor is adjusted to adjust the power supply output voltage so as to compensate for a deviation of the corresponding output resistor from its nominal value, so that the current through the selected output resistor will correspond to its nominal resistance and the nominal voltage of the power supply which is applied to it.

The present invention relates to small current sources and, more particularly, to a small current source which has a plurality of resistors which may be selectively connected in series with a regulated power supply to provide small currents such as pico ampere currents.

' An important object of the present invention is to provide a new and improved small current source in which the output current has a predetermined magnitude when a selected resistor is connected in series with a regulated power supply even though the resistoris a non-precision resistor.

A further object of the present invention is to provide a new and improved small current source in which a regulated power supply is adjusted to compensate for errors in an output resistor when the resistor is connected in series with the regulated output to provide a predetermined output current corresponding to the particular resistor.

Another object of the present invention is to provide a new and improved small current source in which output resistors to be connected in series with the output of a regulated power supply to provide a predetermined current corresponding to the resistor need not be precision resistors Further objects and advantages of the present invention will be apparent from the following detailed description thereof made with reference to the accompanying drawing in which:

FIG.'1 is a circuit diagram of'a current source embodying the present invention; and

FIG. 2 is a circuit showing a regulated power supply suitable for use in the circuit of FIG. 1.

FIG."1 is a circuit diagram of a small current source embodying the present invention. The purpose of the small current source is to provide small currents of known magnitude, such as currents on the pico ampere range; and the source includes a regulated power supply whose output is connected through a selected one of a plurality of resistors 11, 12, 13 to current output terminals 15, 16.

3,399,339 Patented Aug. 27, 1968 ice The power supply has voltage output terminals 17, 18 which are connected across a voltage dividing resistor having a plurality of taps for selecting a range of voltages to be applied to the selected one of the resistors 11, 12, 13.

The output of the power supply is a known voltage, for example, 10 volts, and each of the resistors is intended to provide a predetermined known output current when connected in series with the 10 volt supply. Problems arise in providing small currents at the current output terminals 15, 16 because of tolerances involved in the resistors 11, 12, 13. These resistors may be of the magnitude 10 10 etc. ohms and must be precision if they are to provide a predetermined current for a given output voltage. It is difiicult to obtain precision-resistors in this range and from a cost standpoint it is desirable to use standard non-precision resistors.

In accordance with the present invention, standard non-precision resistors may be used for the resistors '11, 12, 13 and any resistance deviation from the standard is compensated for by connecting a compensating resistor into the power supply when a corresponding one of the. resistors 11, 12, 13 is selected. The compensating resistors adjust the output of the voltage supply to give the predetermined current corresponding to the selected resistor. The compensating resistors are designated by the reference numerals 11a, 12a and 13a. The resistor 11a is set to compensate for the error in the resistance value of the resistor 11, while the resistors 12a, 13a are set to compensate for the errors in the resistors 12, 13, respectively. When the resistor -11 is selected, the compensating resistor 11a is connected into the power supply to vary the voltage output of the power supply to provide the predetermined current for the resistance 11 at the terminals 15, 16, thereby compensating for the error in the resistance of the resistor 11. The resistors 12a, 1301 are similarly used to compensate for the errors in resistance of the resistors 12, 13.

The regulated power supply may be as shown in FIG. 2. The supply shown in FIG. 2 includes a power input transformer 21 having a secondary coil with end terminals 22, 23 and an intermediate tap 24. The terminals 22, 23 are connected to a ground conductor 25, which is in turn connected to the power output terminal 18, through respective diodes 26, 27 poled to conduct current toward the conductor from the transformer. The intermediate tap of the transformer is connected to the conductor 25 by a condenser 28 and to the terminal 22 by a series connected condenser 30 and a diode 31 poled to conduct current from the center tap of the transformer secondary to the terminal 22. The described circuit arrangement charges the condenser 28 on one half-wave with the voltage between the tap 24 and the terminal 23 and on the other half-wave with the voltage between the terminal 22 and the tap 24. The condenser 30' is charged on every other half-wave by the voltage between the secondary terminal 22 and the tap 24.

The positive side of the condenser 28 is connected to the ground conductor 25 while the negative side of condenser 28 is connected to the power output terminal 17 through the collector emitter path of a transistor 40, a load resistor 41 and a diode 42. The base of the transistor is connected to the ground conductor 25 by a resistor 43 and to the negative side of the condenser 28 through the emitter-collector circuit of a transistor 44. The transistors 40, 44 form a Darlington pair and function as one transistor to vary the portion of the voltage across condenser 28 which is applied to the power output terminals 17, -18. The transistors are controlled by a differential amplifier arrangement 45 to maintain a constant potential at the power terminals 17, 18.

The differential amplifier includes a transistor 46 whose base is connected to a junction 47 of a voltage dividing network 48 connected across the power output terminals 17, 18 in parallel with a smoothing capacitor. The voltage dividing network 48 includes a resistor 49 connected between the terminal 17 and the junction 47 and a resistor 50 connected to the junction 47 and to one terminal 52 of a pair of terminals 52, 53, the terminal 53 being connected to the output terminal 18 which is ground. The terminals 52, 53 are the terminals across which the compensating resistors 11a, 12a, 13a are connected when selected to be connected into the power supply circuit.

The collector of transistor 46 is connected to the terminal 17 while its emitter is connected through a resistor 55 to the conductor 25 which is the positive side of the power supply. The resistor 55 also connects the emitter of a transistor 58 to the positive side of the power supply and the collector of the transistor 58 is connected to the output terminal 17 through a resistor 59 and the diode 42. Accordingly the regulated voltage output appearing at terminal 17 is the collector supply for the transistors 46, 58 and the diode 42 provides an additional voltage drop for the collector of transistor 58.

The base of the transistor 58 is connected to the positive side of the power supply by a Zener diode 60, which acts as a voltage reference, and to the negative side of the diode 42 by a diode 61 and resistor '62. The described differential circuit arrangement is such that the voltage of the collector of the transistor 58 will follow the voltage of the junction 47.

The collector of transistor 58 is connected to the base of a transistor 65 having its emitter connected to the negative side of diode 61 and its collectoi connected to the base of transistor 44 through a resistor 67. The negative side of resistor 67 is connected to the negative side of the condenser 28 through series connected resistors 68, 69.

As the potential of the base of transistor '65 varies with the collector potential of transistor 58 the current in its collector-emitter path will vary to in turn vary the potential at the negative side of resistor 67 and at the base of transistor 44 to control the conduction of transistor 44 and in turn the conduction of transistor 40. A Zener diode 70 is connected between the negative side of resistor 68 and the positive side of resistor 41 and provides a collector supply for transistor 65.

The regulation occurs as follows: If the junction 47 becomes more negative with respect to terminal 18 as a result of a voltage change at the power output terminals 17, 18, the conduction of transistor 46 will increase to increase the voltage drop across the resistor 55 to decrease the conductivity of transistor 58 to cause its collector to become more negative. This in turn causes the base of transistor 65 to become more negative with respect to its emitter to increase the conductivity thereof and cause the collector thereof and the base of transistor 44 to change voltage in a positive direction to decrease the conductivity of transistor 44 and that of transistor 40 by reason of the decrease in base current in the latter due to the decreased conductivity of transistor 44. The decreased conductivity of transistor 40 causes the voltage at the power terminals 17, 18 to decrease to maintain the proper voltage at the terminals 17, 18. The converse of the described regulation will occur if the junction 47 becomes less negative with respect to terminal 18.

The voltage maintained at the power output terminals is a function of the ratio of the resistance in the two sections of the voltage dividing network and is adjustable by changing this ratio. In accordance with the invention the compensating resistances are set to adjust the ratio of the resistances in the voltage dividing network to compensate for errors in the resistance of the corresponding output resistor. Consequently, when the output resistor is selected, its corresponding compensating resistor is connected into the voltage dividing network to adjust the output voltage of the regulated supply to provide the predetermined current at the output terminals of the current source. The setting of the adjustable resistances may be made by first measuring the resistance of the output resistors as by applying an ohmmeter and then adjusting the resistances to that value necessary to eifect the compensation of the power supply.

While a particular power supply has been shown and described it will be understood by those skilled in the art that other suitable supplies may be utilized.

Having described my invention, I claim:

1. A small current source comprising a source of unregulated direct current power, a voltage regulating circuit having input terminals connected to said source and output terminals, a series transistor having a collector and emitter path and a base, said collector and emitter path being connected in series between one of said input terminals and one of said output terminals, a differential amplifier having its output connected to said base and a sensing circuit connected across said output terminals, said sensing circuit including a voltage dividing network and a sensing transistor having a base connected to an intermediate junction of said voltage dividing network, a plurality of standard resistors to be selectively and individually connected in series with said output terminals to provide an output current of a predetermined magnitude, said output resistances to have predetermined values to provide a predetermined current, a plurality of compensating resistances each corresponding to a respective one of said standard resistors, means for connecting a predetermined one of said standard resistors into circuit with saidoutput terminals and for connecting a corresponding one of said compensating resistances into said voltage dividing network to provide an output voltage at said output terminals which is compensated for the dilference in resistance of the selected standard resistor from its standard value for providing the predetermined current.

2. A small current source comprising a plurality of output resistors, an unregulated direct current power supply, output voltage terminals, first circuit means comprising a transistor connecting said unregulated power supply to said output terminals, second circuit means including a difierential amplifier connected to sense the voltage across said output voltage terminals and controlling the conduction of said transistor to maintain a constant voltage at said output voltage terminals, selectively operable switching means for connecting a selected one of said output resistors in series with said output voltage terminals and a load, a plurality of adjustable resistors each corresponding to a respective one of said output resistors, said second circuit means comprising a voltage dividing net work for determining the regulated voltage at said output voltage terminals, and switching means for switching the corresponding one of said adjustable resistors into said voltage dividing network when the corresponding output resistor is connected in series with said terminals.

3. A small current source comprising a regulated power supply having a nominal output voltage, a plurality of high resistance non-precision output resistors, each having a nominal resistance, selectively operable means for connecting to the output of said power supply a selected output resistor, a plurality of compensating resistors, and selectively operable means for connecting in said power supply to determine its output voltage a selected compensating resistor which corresponds to the selected output resistor connected to the output of said power supply, each of said compensating resistors being adjustable to adjust the output voltage of the power supplyto compensate for a deviation of the corresponding output resistor from its nominal resistance so as to providev a predetermined output current through the selected output resistor corresponding to its nominal resistance and the nominal voltage from the power supply which is applied to the selected output resistor.

4. A small current source according to claim 3 wherein References Cited said power supply has an impedance circuit therein which UNITED STATES PATENTS includes the selected compensating resistor and which conv trols the output voltage of the power supply. 3-263156 7/1966 Isaacs 2 5. A small current source according to claim 4 wherein 5 3,252,085 5/1966 Lundstrom 323 22 said power supply includes an amplifier having a gain 3,250,981 5/1966 Marks 323-42 which may be varied, and said impedance circuit is a volt- 3,201,680 9/ 1965 Ross et age divider at the output side of said power supply and 2,934,699 4/1960 other having an intermediate point thereon connected to said amplifier to control the latters gain corresponding to the w ORIS RADER "nary Exammer' resistance of the selected compensating resistor which is H. HUBERFELD, Assistant Examiner. connected in said impedance circuit. 

